This invention relates to television systems and more particularly to digital data transmission thereby.
In some presently constituted television systems, digital data signals are transmitted along with the video signal by transmitting this data during one or more horizontal line scan periods during the vertical interval. While this method of transmitting digital data is satisfactory for many applications, it does have its limitations. The vertical interval period is limited to just a few lines per field, on the order of 10 to 15 lines. In the standard NTSC-format, each line is generated in 63.5 microseconds. Since certain equipment to be used to process this information, such as microprocessors are relatively slow, during any 63.5 microsecond period, or one scan line, possibly only four bits per line can be used without the addition of data buffering hardware.
To overcome this data transmission rate limitation, it has been proposed to transmit data during the horizontal blanking interval such that bits of data can be transmitted during each of the horizontal lines of a video scan. In these systems typically a pulse(s) of digital data is transmitted during the horizontal blanking interval and after the chroma burst. While such systems are adequate, they do require an altering of the standard television signal format; namely, the insertion of the data pulses after the chroma burst.
Another method of providing additional information in a television format is disclosed in U.S. Pat. No. 3,723,637. This patent teaches information coding and transmission by amplitude modulating the color burst signal. This system may present a problem in complying with the NTSC standards which specify certain amplitude limitations for the subcarrier signal. It may be difficult to comply with the NTSC standard and still maintain sufficient a.m. modulation for reliable data transmission. Some digital code transmissions may be prohibited because the overall average amplitude of color burst signals will be too low.
Accordingly, it is an object of this invention to provide improved means for data transmission of digital information during the course of a television signal transmission.